The present invention relates to a blow-out prevention device for shutting off an annulus between a drill column and a well wall when an unwanted blow-out of fluid and/or gas from an unstable geological well formation occurs when drilling for oil or gas. From U.S. Pat. No. 4,367,794 and EP patent document Nos. 0,116,443 and 0,205,297 devices are known which prevent an unwanted blow-out of fluid and/or gas from an unstable geological well formation when drilling for oil or gas. These known devices for shutting off a fluid and/or gas blow-out have considerable disadvantages. U.S. Pat. No. 4,367,794 concerns an acoustically activated blow-out prevention device which, by means of a motor-activated, movable internal sleeve in a valve body and a flap valve, enables the drilling fluid to circulate out and the annulus between the drill column and the well wall to be shut off. Communication between the sealing device and the annulus is via doors and ducts in the valve body and chokes in the sleeve. The disadvantage is that the seals between the sleeve and the valve body are subject to too much erosive wear on account of the high differential pressure. EP patent document No. 0,116,443 concerns a blow-out prevention device which is activated when a preset differential pressure arises between the annulus pressure and the internal pressure in the drill column. This differential pressure controls a slide valve which is mounted in the valve body. The disadvantage is that the slide valve can easily become stuck and that the seals are subjected to erosive wear. EP patent document No. 0,205,297 concerns a blow-out prevention device in which a solenoid valve controls the pressure to a ball check valve which alters the circulation pattern of the drilling fluid. Activation is by pressure waves being sent through the drilling fluid in the drill column. The disadvantage of this arrangement is that there are at least three valves and that there is, therefore, a certain risk that one or more valves might become stuck or might leak. Shallow, thin gas and/or fluid reservoirs under high pressure represent one of the most serious problems when drilling for gas or oil. Shallow gas is gas which is located in the upper part of a sedimentary geological formation and is usually 200 to 800 meters below the sea bed.
These gas and/or fluid reservoirs are usually 2 to 6 meters thick and often consist of unconsolidated sand with high porosity and permeability. The extent of these reservoirs can be great and the probability of an uncontrolled blow-out can be high, with a correspondingly high risk of well damage. The formation pressure in the upper layer is usually low. To prevent reservoir fluid penetrating into the well, the hydrostatic weight of the drilling fluid column must be higher than the pressure in the reservoir, but not so high as to risk the well wall cracking. If this happens, the drilling fluid located in the drill hole might leak out into the formation and an uncontrolled blow-out might take place as a consequence of the reduced height and thus reduced hydrostatic weight of the drilling fluid column. To increase safety, a blow-out prevention device down in the hole can be used to shut off the annulus between the drill column and the well wall above the unstable, critical reservoir layer. Thereafter, the fluid or gas located above the valve circulates out to the surface and the annulus is filled with fluid which has sufficient specific weight to withstand the reservoir pressure.